Highcharts iOS wrapper provides its own colors implementation. As you can notice, some options are of HIColor type. You can instantiate the desired color in few ways which are described in the API documentation

In here, we will show the most complex case which is gradient usage. For example, you can instantiate a color for chart background:

HIChart *chart = [[HIChart alloc]init];
chart.backgroundColor = [[HIColor alloc]initWithLinearGradient:@{ @"x1": @0, @"x2": @0, @"y1": @0, @"y2": @300 } stops:@[
    @[@0, @"rgb(102, 153, 161)"],
    @[@1, @"rgb(128, 135, 232)"]

Original info at: the Highcharts iOS GitHub repository